Browsing by Author "Okada, N"

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Brane world cosmological solution to the gravitino problem
(2005-01-01)
Okada, N
;
Seto, O
;
物理研究所
;
Institute of Physics
We investigate the thermal production of gravitinos in the context of the brane world cosmology. Since the expansion law is modified from the one in the standard cosmology, the Boltzmann equation for the gravitino production is altered. We find that the late-time gravitino abundance is proportional to the "transition temperature", at which the modified expansion law in the brane world cosmology is connecting with the standard one, rather than the reheating temperature after inflation as in the standard cosmology. This means that, even though the reheating temperature is very high, we can avoid the overproduction of gravitinos by taking the transition temperature low enough. Therefore, the gravitino problem can be solved.
Neutralino dark matter in brane world cosmology
(2005-03-01)
Nihei, T
;
Okada, N
;
Seto, O
;
物理研究所
;
Institute of Physics
The thermal relic density of neutralino dark matter in the brane world cosmology is studied. Since the expansion law at a high energy regime in the brane world cosmology is modified from the one in the standard cosmology, the resultant relic density can be altered. It has been found that, if the five-dimensional Planck mass M-5 is lower than 10(4) TeV, the brane world cosmological effect is significant at the decoupling time, and the resultant relic density is enhanced. We calculate the neutralino relic density in the constrained minimal supersymmetric standard model and show that the allowed region is dramatically modified from the one in the standard cosmology and eventually disappears as M-5 is decreasing. We also find a new lower bound on M-5 greater than or similar to 600 TeV based on the neutralino dark matter hypothesis, namely, the lower bound in order for the allowed region of neutralino dark matter to exist.
Relic density of dark matter in brane world cosmology
(2004-10-01)
Okada, N
;
Seto, O
;
物理研究所
;
Institute of Physics
We investigate the thermal relic density of cold dark matter in the context of brane world cosmology. Since the expansion law in a high energy regime is modified from the one in the standard cosmology, if the dark matter decouples in such a high energy regime its relic number density is affected by this modified expansion law. We derive analytic formulas for the number density of dark matter. It is found that the resultant relic density is characterized by the "transition temperature" at which the modified expansion law in brane world cosmology is connecting with the standard one, and can be considerably enhanced compared to that in the standard cosmology, if the transition temperature is low enough.